Because something with a cryptographic hash, like sha256, you can do millions (or in some cases billions) of hashes per second on the same GPU.

Password hashes, like bcrypt, PBKDF2 and scrypt, are massively slower. That doesn't mean they're uncrackable, it just means they are expensive to crack, so a strong password in a well implemented password hash will take a long time (and cost a lot of money) to crack, by which time the user has hopefully noticed and had time to change their password.

Additionally, there's a new player in town called Argon2 that attempts to solve the problems with bcrypt and scrypt. It's the most recent winner of the password hashing competition.

https://github.com/P-H-C/phc-winner-argon2

What problems with bcrypt/scrypt does argon2 solve?

bcrypt is limited to 72 characters (admittedly this is not that big a practical limitation), and scrypt is an excellent KDF, but wasn't designed as a password hash, and thus doesn't have great GPU defence. In practice, they're still both great, but that doesn't mean we should stop trying to do even better.

Argon2 looks really interesting, but it is still relatively new.

If someone isn't ready to go with Argon2 and truncating past 72 characters is an issue, Django (and possibly other implementations that I'm not aware of) overcome that by first hashing the password with SHA256 before passing it to bcrypt.

> scrypt is an excellent KDF, but wasn't designed as a password hash, and thus doesn't have great GPU defence

Last time I heard of scrypt, it was being used in Litecoin, a fork of bitcoin, because it was preventing GPU based crackers.

I don't know if it has anything to do with GPUs, I had thought it was due to memory, as referenced on the wiki page for Litecoin: "Litecoin uses scrypt in its proof-of-work algorithm, a sequential memory-hard function requiring asymptotically more memory than an algorithm which is not memory-hard."

It rate limits via IO (specifically memory IO) rather than processing power (CPU/GPU).

Ah, that's interesting enough to be worth looking into.

Yes if your password looks like: z1c4KzC*ITtt4Ka%vVDeei!aFSuEOTiDFCJ2NXc8Z7Jz then we are looking at enough time for you as a user to not care.

When you have a massive leak, it's better to have under 10 angry users about that they have been hacked (increasing at a slow pace) than most of your userbase raging at you and leaving your service.